Joseph Henry Downs, 72, a retired White House policeman who was shot and seriously wounded in the attempted assassination of president Harry S. Truman at Blair House in 1950, died Thursday at the home of a son in Baltimore after apparently suffering a heart attack.

He was in plain clothes and on duty at Blair House, where the Trumans were living while the White House was undergoing renovation, when two Puerto Rican Nationalists tried to shoot their way into the residence on Nov. 1. Mr. Truman, who was napping inside, was not injured.

Mr. Downs was shot in the chest, abdomen and thigh after he raced from the Blair House kitchen down an alley to the scene of the Pennsylvania Avenue gun battle.

A fellow officer, Leslie Coffelt, and one of the gunmen, Greselio Torresola, died in the exchange of gunfire, another White House policeman, Donald T. Birdzell, was wounded. The second gunman, Oscar Collazo, was convicted and sentenced to die. President Truman commuted his sentence to life imprisonment.

Mr. Downs was hospitalized at the old Emergency Hospital, nearby on New York Avenue. He returned to duty on Jan. 15, 1951, with an elevation in rank from private to corporal.

There were other honors for his bravery, including a medal and certificate from Truman and an American Legion medal of honor.

Mr. Downs ended a 27-year career with first the Washington police force and then the White House police force, when he retired in 1956.

He then worked for the U.S. Customs Service in Miami, Fla., until transferring to the Baltimore offices in 1970. He retired for a second time in 1973.

Born in Cincinnati, Mr. Downs left school at the age of 17 to serve in the Marine Corps. He was wounded during a Nicaraguan revolt in 1925 but completed four years with the Marines. He joined the Washington police force in 1929 and became a White House policeman 10 years later.

He is survived by his wife, Mary R., and a son, John F., of the home in Baltimore; three other sons, Joseph M., of Silver Spring, and James E. and William R., both of Baltimore, and 11 grandchildren.